In the papers the past few weeks, there have been adverts for a product called Onspeed, which, the blurb tells you, will improve your internet connection’s speed by up to 5 times. It does so by directing all your data through their own servers, which compress the data before sending it on to you. However, this faster connection comes at a cost, which is sacrificing quality, which the press adverts don’t really tell you.

First of all, a quick layman’s guide to how compression works. This is not intended to be a comprehensive coverage of the state of the art, and technical-types will probably pick holes at this, but the gist of it is correct. I hope.

Compression of any data is based upon stripping out redundant information. This can be done in a number of ways. For example, in a typical English text there are more incidences of the letter ‘e’ than there are of the letter ‘z’. However, a computer will usually devote the same amount of space – one byte, which is eight bits – to storing it. However, as it’s more likely an ‘e’ will crop up in the text, we could come up with a system where we only use two bits to represent an ‘e’. This means we’ve saves 75% on our ‘e’s.

This is one way of compressing text (ZIP files are partly based on this concept), and it’s especially useful as it’s lossless – we don’t lose any information in the text. However, we could also compress text by, stripping out all the vowels, making it shorter. Ths sntnc s n xmpl f sch cmprssn tchnq.

This isn’t very useful for text, as we have lost some information (the vowels), and it becomes trickier for us to decipher. But, this does work for other media, like pictures and music, where we’re less discriminating. Pictures often have pixels that are only slightly different, but look quite similar to each other, so we can bunch them all together. Audio files have a lot of frequencies we can’t hear very well, so we can remove them without too much fuss. These are lossy formats of compression (like JPEG and MP3) – we sacrifice some quality for better compression. The more we want to compress, the more information we remove, so the poorer the quality. Thus heavily compressed JPEGs and MP3s will look and sound a bit shitty (e.g.). Usually the data is then encoded in a lossless format, for further compression goodness.

The end result is compressed data, which, as it has already been compressed, has very little redundant data in it. Which means that trying to compress it again in a lossless manner will usually result in very little further gain (try it yourself, use WinZip or whatever to compress a JPEG or MP3 file).

So Onspeed will definitely work for uncompressed data – such as the text in webpages, emails, Word and Excel documents – and will probably do so very well. However, most of the more sophisticated data – images such as GIF and JPEG, Flash applets, music in MP3 format, and movies in MPEG or Real format, will have already been compressed, and so further lossless compression will not come to much.

Onspeed does operate on many compressed formats (mostly graphical) as well, with promises of more to come. The only way they can do this is to perform further lossy compression on the data, reducing the quality even further.

While Onspeed do discuss the loss of quality in the technology and support sections of their website in some detail, and the software allows you to adjust the level of compression, their press adverts totally skirt the issue; while talking up their broadband-level speeds, they stay well clear of any discussion of what happens to the quality. Caveat emptor should apply here – although you will get broadband-level speeds on a dialup account for cheap, the quality of graphics will not usually match it.

3 Responses

Manda

I didn’t spot anything about jpegs, but they do say: “ONSPEED will not accelerate certain files such as MP3, Mpeg, EXE and streaming video, these files will pass through our servers without being accelerated.”

Although they also say that by mid-2004, MP3 and MPEG video files will also be compressible, though there is no confirmation that they’ve succeeded yet.

Manda

So not only is it out of date by about half a year, it’s also contradictory.

Caveat emptor, indeed.

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